A thin circular ring of radius R carries total charge q uniformly. A point charge Q of the same sign is placed at the centre. Neglect deformation of the ring. The increase ΔT in the tensile force in the ring is:
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Question

A thin circular ring of radius R carries total charge q uniformly. A point charge Q of the same sign is placed at the centre. Neglect deformation of the ring. The increase ΔT in the tensile force in the ring is:

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G1 Physics · Accepted Answer

The linear charge density is λ = q/(2πR). The outward electrostatic force per unit length on the ring due to Q is f = kQλ/R². For a small ring element subtending dφ, radial equilibrium gives ΔT·dφ = fR·dφ. Hence, ΔT = fR = kQλ/R = kQq/(2πR²).